To produce a plurality of gear steps, modern automatic transmissions of motor vehicles usually consist of a number of planetary gearsets, which can be in the form of simple planetary gearsets and/or composite planetary gearsets, such as a Simpson gearset or a Ravigneaux gearset. The components of these planetary gearsets such as the sun gears, ring gears and carriers (planetary carriers) are, in some cases, coupled in a rotationally fixed manner to one another or, by means of associated frictional shifting elements, preferably made as disk clutches and disk brakes, they can be coupled to an input shaft, an output shaft or to one another, or locked relative to the transmission housing, in a selective manner, i.e., in each case in order to engage or disengage a particular gear step.
However, owing to the requirement that the drive engine should operate, as much as possible, in a speed range in which the specific fuel consumption is low, the number of gear steps is continuously increasing and this necessarily entails increasing the number of planetary gearsets that must be provided in an automatic transmission. But with the usual axially adjacent arrangement of the planetary gearsets, this results in greater axial structural length of the automatic transmission, such that, particularly in a motor vehicle, a front transverse installation of the drive unit consisting of the drive engine and the automatic transmission is made more difficult or even prevented.
Accordingly, several designs for multi-stage automatic transmissions with at least two simple planetary gearsets have been proposed in which one of the planetary gearsets is arranged coaxially with and radially inside the sun gear of the other planetary gearset. In this way the two planetary gearsets can be arranged in a common radial plane or in radial planes that are only slightly axially offset relative to one another so that there is one less gearset plane, compared with those in an axially adjacent arrangement, and the axial length of the respective automatic transmission is reduced.
A first automatic transmission, with a radially nested arrangement of two simple planetary gearsets, is known from U.S. Pat. No. 5,429,557 A. The two planetary gearsets are arranged coaxially over the output shaft and form the main gearset of this known automatic transmission. The ring gear of the radially inner planetary gearset and the sun gear of the radially outer planetary gearset are formed integrally, i.e., combined in a single, common transmission component. The planetary carriers of the two planetary gearsets are solidly connected to one another and are in driving connection with the output shaft.
In a first embodiment of this automatic transmission, six forward gears and one reverse gear can be engaged by means of three shift clutches and two shift brakes such that a driving connection, on the input side to the input shaft arranged axis-parallel to the output shaft, can be produced, respectively, by a first input stage with an underdrive transmission ratio and/or by a second input stage with an overdrive transmission ratio. In a second embodiment of the automatic transmission, six forward gears and one reverse gear can be engaged by means of two shift clutches and three shift brakes such that the input-side driving connection is produced by a single input stage with an individual transmission ratio and, if necessary, by a simple planetary gearset connected upstream from the main gearset.
A second automatic transmission, with a radially nested arrangement of two simple planetary gearsets, is described in a number of embodiments in DE 10 2005 014 592 A1. This known automatic transmission comprises four simple planetary gearsets coupled with one another. Two of the planetary gearsets are arranged in a common radial plane with one of the two planetary gearsets arranged radially inside the sun gear of the other of the two planetary gearsets of the upstream gearset. The ring gear of the radially inner planetary gearset and the sun gear of the radially outer planetary gearset are integrally coupled with one another. In this known automatic transmission, eight forward gears and one reverse gear can be engaged by means of three shift clutches and two shift brakes.
A further automatic transmission, with a radially nested arrangement of two simple planetary gearsets, is the object of a previously un-published DE 10 2008 000 429.4 by the present applicant. A number of embodiments of the automatic transmission described therein comprise, in each case, an upstream gearset and a main gearset. Whereas in all the embodiments, the upstream gearset consists of two coupled simple planetary gearsets, the main gearset is formed either as two coupled simple planetary gearsets or as a Ravigneaux gearset.
The two planetary gearsets of the upstream gearset, in some embodiments, are arranged in radial planes that are slightly axially offset relative to one another and, in other embodiments, are in a common radial plane, whereby the first planetary gearset is arranged inside the sun gear of the second planetary gearset. The ring gear of the radially inner planetary gearset and the sun gear of the radially outer planetary gearset are solidly connected to one another.
The carriers of the two planetary gearsets are also solidly connected to one another and, depending on the embodiment concerned, are coupled in a rotationally fixed manner to the ring gear of the first planetary gearset of the main gearset, to the small sun gear of the Ravigneaux gearset, or to the large sun gear of the Ravigneaux gearset. With each embodiment of this automatic transmission, nine forward gears and one reverse gear can be engaged by means of three shift clutches and three shift brakes such that to avoid drag torques of disengaged frictional shift elements, in each case, one shift clutch and one shift brake are made as claw couplings.
In the automatic transmissions described, the rotationally fixed coupling of the ring gear of the radially inner planetary gearset to the sun gear of the radially outer planetary gearset is, in each case, produced by making the two components integral, i.e., combining them in a common transmission component. In DE 10 2005 014 592 A1, however, it is mentioned that the ring gear and the sun gear concerned can also be made as separate components which, in a manner not described further, can be connected to one another either solidly rotationally fixed or, for the sake of acoustic decoupling, via an elastic intermediate layer made from an elastomeric plastic which allows a limited amount of elastic relative rotation.
A design, known per se and illustrated in the sectional view shown in FIG. 3 of DE 10 2005 014 592 A1, for axial and radial mounting and for the driving connection of the gearwheels connected integrally to one another, namely, the ring gear of the radially inner planetary gearset and the sun gear of the radially outer planetary gearset, consists in that the common transmission component is connected, by means of driving teeth, to a disk-shaped carrier. In turn, the carrier is mounted by means of axial bearings relative to axially adjacent components and by means of a radial bearing on the centrally arranged input shaft of the automatic transmission concerned, and is in driving connection with a shift clutch arranged outside the area shown in the figure. The driving teeth consist of a set of inner teeth formed on the gearwheel side by the axially extended teeth of the ring gear of the common transmission component and a corresponding set of outer teeth arranged on the carrier side on the radially outer rim of the carrier. By means of a circlip held in a ring groove arranged close to the axial outer rim of the inner teeth on the carrier side, the common transmission component, namely, the combined sun gear-ring gear, is axially guided at least in one direction.
Since, owing to the tooth geometry, the outer teeth of the sun gear of the radially outer planetary gearset are loaded more severely in terms of surface pressure than the inner teeth of the ring gear of the radially inner planetary gearset, the sun gear usually has to be hardened whereas for the ring gear, in contrast, a tempering treatment, for example, by nitrocarburizing, would be sufficient. But if the integrally made common sun gear-ring gear transmission component is hardened, this necessarily leads to some distortion which makes it necessary to carry out mechanical finish-machining of the ring gear teeth, for example, by grinding. Disadvantageously, this entails more production effort and correspondingly increased costs.
A further disadvantage is that the outer teeth of the carrier have to be helically geared, with higher production cost and effort since the teeth of the planetary gearsets and also the inner teeth of the ring gear of the common sun gear-ring gear transmission component, used as driving teeth, are usually helically geared in order to avoid louder gearing noise.